Lemma:
If gcd(a,m)=1, then there is a unique inverse to a mod m. That is there is a unique solution to: (mod m)

Proof
By the Euclidean Algorithm, you know there exist integers x and y such that

ax + my=1

Reduce mod m and we see (mod m)
as desired.

Uniqueness of this solution mod m comes from solving this diophantine equation. There are an infinite number of solutions to this equation ax +my = 1 if (a,m)=1 but they should all be congruent mod m. Alternatively if you know what a group is this is just the multiplicative group of units for the integers modulo m and as we all know inverses in a group are uniqe.

To solve your question, simply take the unique inverse that I found above and multiply it by b, multiplication is well defined, so this is the unique answer to the equation (mod m)